One common way of moving the focus around in a user interface on Windows is to use the Tab key to move sequentially between controls. The contents of a WPF tool-bar seem to act like a kind of tabbing black-hole. The focus goes in there but never comes out again. Try pasting this Xaml into XamlPad to see the problem:

Xaml Code (.NET 3.0)
<DockPanel>
  <ToolBar DockPanel.Dock="Top">
    <Button Content="B"
            Command="EditingCommands.ToggleBold" />
    <Button Content="U"
            Command="EditingCommands.ToggleUnderline" />
    <Button Content="I"
            Command="EditingCommands.ToggleItalic" />
  </ToolBar>
  <RichTextBox />
</DockPanel>

Fortunately this can be easily fixed by using the KeyboardNavigation attached property and changing the TabNavigation to "Continue". This allows the tab focus to move out of the toolbar and back to the other elements of the UI.

Xaml Code (.NET 3.0)
<DockPanel>
  <ToolBar DockPanel.Dock="Top"
           KeyboardNavigation.TabNavigation="Continue">
    <Button Content="B"
            Command="EditingCommands.ToggleBold" />
    <Button Content="U"
            Command="EditingCommands.ToggleUnderline" />
    <Button Content="I"
            Command="EditingCommands.ToggleItalic" />
  </ToolBar>
  <RichTextBox />
</DockPanel>